Knowledge of the fine structural organization, molecular composition and permeability properties of the cell surface of intestinal protozoan
cysts is important to understand the
biologic basis of their resistance. Recent studies on the biology of the
cyst walls of Entamoeba histolytica and Entamoeba invadens have considerably advanced knowledge on the cellular processes involved in the transport and surface deposition of the main
cyst wall components. Using transmission electron microscopy, cytochemistry, scanning electron microscopy and freeze-fracture techniques, we have obtained new information. In mature
cysts the permeability of Entamoeba
cysts is limited to small molecules not by the
cyst wall, but by the plasma membrane, as demonstrated with the use of
ruthenium red as an electron-dense tracer. Cell walls of E. histolytica
cysts are made up of five to seven layers of unordered fibrils 7-8 nm thick.
Alcian blue stains a regular mesh of fibrils approximately 4 nm thick, running perpendicularly to the
cyst wall. In addition, abundant ionogenic groups are seen in
cyst walls treated with
cationized ferritin. In the mature
cysts of E. histolytica and E. invadens small cytoplasmic vesicles with granular material were in close contact with the plasma membrane, suggesting a process of fusion and deposition of granular material to the cell wall. The plasma membrane of mature
cysts is devoid of intramembrane particles when analyzed with the freeze-fracture technique. When viewed with scanning electron microscopy the surface of E. histolytica
cysts clearly differs from that of Entamoeba coli and E. invadens.